“It does no harm to the romance of the sunset to know a little bit about it.” -Carl Sagan

I would argue the exact opposite, in fact: the beauty of a sunset, in all of its varieties and variations, is only enhanced the more you know about it.

Image credit: Dan Schroeder, via Picasa.

The next time you watch the Sun descend through the sky, towards the horizon, you might marvel at how the Sun remains the same size all the way down. At just slightly over half-a-degree, the Sun appears to drop at a constant rate throughout the afternoon and into early evening.

But there are some small changes that are extremely important if you want to understand the beauty behind the sunset.

Image credit: Tamas Ladanyi (TWAN), over Lake Balaton, Hungary.

The first and most obvious is the change in coloration of the Sun, as well as a severe drop in the Sun’s brightness. On an airless world like the Moon, the Sun at sunset would look no different than at any other time. But it’s the Earth’s atmosphere that makes sunsets so special.

Image credit: Bob King of http://astrobob.areavoices.com/.

When the Sun appears progressively lower and lower on the horizon, its light needs to pass through more and more of the atmosphere to reach our eyes. You might not think of the atmosphere as being a very good prism, but when you pass through around 1000 miles of it just before the Sun dips below the horizon, it starts to add up.

Image credit: Pete Lawrence (Digital-Astronomy).

The bluer wavelengths of light get scattered away, leaving only the reddest wavelengths that reach your eye. As the sun drops towards the horizon, it progressively loses violets and blues, then greens and yellows, and finally even the oranges, leaving only the reds behind.

You may not even realize it, but by time you’d see a sunset like the picture above, the Sun has already technically set, it’s only due to the fact that the atmosphere bends light that we’re still seeing it like this.

Image credit: R Nave of Hyperphysics.

This is why, if you time a sunset, it will take longer than the expected 120 seconds to go from the moment it touches the horizon to the moment it dips below, even during the equinox at the equator, where it rises and sets as close to completely vertical to the horizon as possible. The Sun appears to linger due to the refraction of our atmosphere.

Also, despite its red appearance, there really still is blue and green light coming from the Sun, of course, while this is going on. But these shorter (i.e., bluer) wavelengths refract slightly more than the lower frequency ones, meaning that the reds come in at a different, shallower angle than the greens and blues, that come in at a slightly steeper angle.

Image credit: R Nave of Hyperphysics.

Given a clear path to the horizon — such as over the ocean — this means that there’s a slight region of space just above the reddened Sun where only the shorter wavelength light is visible!

And when that happens, in addition to the normal color gradient that comes with a sunset, you can also get a small, separate region above the disk of the Sun that appears yellow, green, or even blue!

This optical phenomena is always most clearly visible over a flat area in pollution-free skies, and is known as the green flash. It occurs in many different stages, sometimes appearing at the limb of the Sun or just above it, but most it commonly appears just after the disk of the Sun has set, in a literal “flash” lasting just a few seconds, just barely above the horizon.

Image credit: Emil Ivanov, in Tel Aviv.

Although there’s a lot of green light in the Sun, the bluest wavelengths refract even more than the green ones do. In principle, you could get a “flash” of any wavelength — yellow, green, blue, or even violet — if the atmosphere cooperated. Although green and yellow flashes are the most common, under just the right atmospheric conditions, you can see even blue colors flashing at a high angle above the top of the Sun!

Image credit: Mario Cogo.

This applies to any very bright, white-light object that encounters our atmosphere as seen just barely above the horizon. So that means the Moon, which reflects sunlight back at us, should also exhibit a green flash under the right atmospheric conditions. And although I’ve never seen it with my own eyes, some diligent astrophotographers have captured the sight to share with us all.

Image credit: Laurent Laveder (PixHeaven.net / TWAN).

You may be wondering, if greens and blues appear slightly above the disk of the Sun (or Moon), could we ever see a red flash slightly below the disk?

Under just the right, favorable atmospheric conditions, that’s exactly what happens!

Image credit: Stefan Seip.

Way back on the old blog (some four years ago), I posted a short explanation of the green flash, and little did I know that years later, I would receive the following message from Don Arnold of Chattanooga, TN:

I thought this was a hoax every time I visit Costa Mesa..so last week we were on the costa mesa pier and had my good Nikon set the motor drive to max and took 30 frames right at sunset. So I think I have a good one. You will have to zoom in but it looks good…thanks for the great explanation on this!

Here was the image he enclosed.

Image credit: Don Arnold.

And here is the zoomed-in-version (my apologies for my lousy image processing skills):

Image credit: Don Arnold.

The sunset is beautiful to anyone’s eyes, and the clarity or dustiness of the horizon, the quality and turbulence of the atmosphere, and position of the Sun give us a great diversity of beautiful sights.

But when you see a color gradient on the Sun, a red lip at the bottom, or a yellow, green, or blue rim at or above the top, will you see less beauty or more for having read and understood this? To me, at least, everything is more beautiful the more you know. Thanks for sharing the beautiful physics of sunsets with me!

Comments

I love watching sunset at the McMath-Pierce Solar Telescope with Astronomy Camp. By making the image of the sun 1 meter across, there are so many subtle phenomena that you can finally see! My favorite, though, is how sunspots appear, which is an exercise for the reader to consider…

I was wondering this today- if the larger amount of atmosphere at the horizon that light must go through is enough to change the color of the sun while it’s rising/setting, do our observations of other stars at the horizon go through a similar effect? I’ve never seen other stars change color as they rise and set, but is this an effect that’s only visible with special equipment?

I photograph sunset quite frequently and have captured a wide variety of phenomena including green, blue and purple flashes (sometimes more than one color at a time), mock mirage flashes, cloud top green and blue flashes and omega sunsets. One of the common myths about green flashes is you need to be watching sunset over the ocean, but most of mine have been photographed over mountains in Tucson (and some over the ocean during my travels).

You can check out my photos on my blog at http://halfastro.wordpress.com/ Just type “green flash” or “sunset” into the search box and you will find lots of pics.

Stars aren’t (apart from the very brightest) bright enough to show colour: our eyes just don’t collect enough photons.

Given the stars are dimmer the closer they are to the horizon and that the colouration effect is stronger closer to the horizon, the effect will require more light gathering than your eyeball can manage.

With regard to your Sagan quote and your comment in response to it, I think it’s clear that you’re (deliberately and humorously in irony?) misinterpreting his intended meaning.

That is, he’s implicitly contrasting knowing “a little” with knowing nothing, while you are inferring that he was contrasting knowing “a little” with knowing “more than a little” and are therefore contradicting him.

This is a nice example of linguistic pragmatics. With that quote alone and nothing else known about Sagan or anything he wrote, we don’t know if the harm he’s denying is relative to knowing more, or knowing nothing. (He could be denying it relative to both, too!) So we’d then probably first ask ourselves if this is in any way idiomatic: is “know a little about X” typically used to contrast against knowing more, or knowing nothing? On that basis, I think native speakers of American English would assume that it’s contrasted against knowing nothing, because an intended contrast against knowing more would have taken the form “know little about X”, eliding “a”.

But Ethan and most readers of this blog are aware that Sagan was a scientist, an astronomer. If we were looking for more specific context and Sagan had been a nineteenth century romantic poet, we might infer that he was contrasting against knowing more. However, as I wrote, we know that Sagan was a scientist, indeed an astronomer who had a particular interest in science of the appearance of heavenly bodies, and so this additional context argues that he was contrasting against knowing nothing.

“The next time you watch the Sun descend through the sky, towards the horizon, you might marvel at how the Sun remains the same size all the way down.”

I’m surprised that no one has commented to deny this, as there’s (still?) very many people who believe that the Sun and Moon (primarily the Moon, though) optically enlarge at the horizon — that is, they believe that it’s magnified. Many years ago, in attempting to explain this illusion to others, I coined for my purposes the expression “perceptual illusion” as distinct from “optical illusion”. (By no means am I the first to use this nomenclature; unfortunately, it’s not really caught on.)

Interestingly, in at least one case I found that someone was incapable of comprehending that distinction. I’m not sure what that implies.

Anyway, I find the fact that the Moon (or Sun) isn’t optically larger near the horizon yet appears so to be more interesting than the folklore that it’s magnified. Not unlike the also false “glass is a liquid that flows in old windows” folklore, the truth is more interesting and so is the implicit sociology of the folklore.

The origin of the redness of the sun at sunrise and sunset have actually been experimentally verified to be attributable to a frequency shift towards red characterized by the loss of energy by light to a cold medium without any relative motion between the source. (Isoredshift)

Which makes sense, as we know (but for some reason tend to ignore) red light is absorbed by our atmosphere. Water is blue because light blue light scatters in water while red light is absorbed at very shallow depths. If red light has problems making through our atmosphere when the sun is at zenith (shallowest depth), why do we expect if we swim deeper (sun at the horizon) the blue light eventually “scatters away” and the red light “un-absorbs” itself back into existence?

Here is a link to experimental verification that light shifts frequency depending on the medium it is propagating through.

There’s various proposed explanations for the horizon effect — the most convincing (because it’s well demonstrated in other contexts) is our perceptual apparatus which determines relative size in the context of parallax and nearby objects of known size. But the last time I read about this, it wasn’t entirely convincing (as the horizon effect still seems to work when there aren’t other objects in the visual field besides the horizon). It’s most likely an erratic and synergistic perceptual artifact with no simple, universal explanation.

Bear in mind that the perceptual apparatus you’re talking about is still working even if there are no reference objects, based solely on your mental model of the horizon vs the sky — and in that model, the sky is closer than the horizon. So our brains “know” that if something is in the sky with the same angular size as something at the horizon, then the thing at the horizon must be bigger. And thus it appears so, because your brain loves to do you favors like this.

By the way, the effect on stars at the horizon can be really awesome. The twinkling becomes super exaggerated and shifts through many colors (though red appears more often). Sometimes I like to take the eyepiece out for a trippy kaleidoscope effect.

@ Keith
“That is, he’s implicitly contrasting knowing “a little” with knowing nothing, while you are inferring that he was contrasting knowing “a little” with knowing “more than a little” and are therefore contradicting him.”

No, he’s agreeing with Sagan completely. If knowing a little about the sunset does no harm to its romance (compared to knowing nothing), then knowing a lot doesn’t harm it either. And Ethan says it’s even the opposite of harming the romance: The more you know about the sunset, the more you can appreciate its beauty.

I had some trouble understanding your argument until I realized that there’s ambiguity about what Ethan is opposing.

Your argument is that Ethan is asserting that he would not only assert the contrary, but the contradictory (which is where “opposite” comes in) against the same implicit argument (call it assertion Y) that Sagan is arguing against (as opposed to Ethan arguing against Sagan’s quoted statement itself, call it “assertion X”).

I think that’s what Ethan intended and the colon supports this (although even if that were not his intention, the colon wouldn’t have been out-of-place or changed how his sentence was parsed, so it’s ambiguous). All the ambiguity arises from the fact that sentence in relationship to the quote makes it unclear whether Ethan is “opposing” the quote itself, or the implicit claim that the quote also refutes.

I didn’t see this at all when I first considered this and now I think it’s an even more interesting case in pragmatics than I had already thought.

“It does no harm to the romance of the sunset to know a little bit about it.” -Carl Sagan

“I would argue the exact opposite” -Ethan

Wouldn’t the “exact opposite” imply that knowing a little bit about sunsets DOES harm the romance? To my parsing, Ethan seems to want to say that Sagan understated the issue, which is not “the exact opposite.” I don’t see that the colon affects the interpretation at all. It reads like a logic error. I found it quite jarring, since it was obvious Ethan was amplifying Sagan, not disagreeing with him.

(But you know, blogs are not really meant to be paragons of writing excellence.)

“Wouldn’t the ‘exact opposite’ imply that knowing a little bit about sunsets DOES harm the romance?”

No. It’s the opposite of “knowing a little bit about sunsets harms the romance” which Sagan disputed, but did not go so far as to state the opposite. He simply said it is not the case that it harms. Ethan then took it farther by stating the opposite — that it helps.

Which is of course made explicit by the second part of the sentence. The colon itself doesn’t mean anything. The context of the rest of the sentence that follows the colon does.

It may be ambiguously worded, but ‘ambiguous’ means ‘open to multiple interpretations’. When one of the ways to parse the first part of the sentence is nonsense and contradicts the rest of the sentence, and another way to parse it makes perfect sense and matches the rest of the sentence AND is what you knew he meant in the first place, then the decision to choose the first one is just baffling.

The concept stated is very interesting. I never before thought of the change in color of the Sun being relative to the distance between a person and the Sun. However, it makes sense as the Sun’s rays must travel through more of the atmosphere (meaning more Air particles) and alter the path of the Sun’s waves. What really interested me about this post was the fact that when the sun is fully set, it has actually already passed. Not only does the atmosphere make up for the changes in color of the sun, but it also bends the path of light so that the Sun is still visible after it has already passed.

No offense to Ethan, since his meaning is clear from the rest of his post, but by saying that he argues the exact opposite of Sagan’s quote is inconsistent with the remainder of his article. The quote can be translated into sentential logic as A—>~B, where A = One knows a little bit about the sunset, and B = Harm is done to the romance of the sunset. Roughly, the English translation is If one knows a little bit about the sunset, then no harm is done to the romance of the sunset. I think that caputures adequately the meaning of Sagan’s quote.

Now, the exact opposite of that quote would be ~(A—>~B). Since any conditional, p–>q is logically equivalent to ~p V q, the original quote is equivalent to ~(~A V ~B). By DeMorgan’s law, this is equivalent to A and B (sorry, I don’t know how to type the normal “and” symbol). In English, this translates to We know a little bit about the sunset and the romance of the sunset is harmed, which is clearly not the meaning intended by Ethan in his post.

I feel like my above post is quibbling over a very minor point. Your meaning was very clear from the context of the post, so I would not have ordinarily posted something like I did above. I only did so because it seemed to be a topic of discussion in the comments. I had no intention of criticizing your writing.

It’s really not helpful to try to make authoritative, absolute logic-based parsings of natural language because, for example, it’s ambiguous what “exact opposite” is intended to mean.

That said, Wow makes a logical fallacy, “denying the antecedent”. His argument can be understood as “If P, then Q therefore if not-P, then not-Q”, where “knowing a little” as P and “more romance” as Q:

If knowing a little [P] about the sunset makes the romance of it more intense [Q]
therefore
If NOT knowing a little [not-P] about the sunset DOES harm it [not-Q].

That’s a fallacy because things other than P can result in Q and therefore the denial of P does not necessarily result in the denial of Q. In contrast, the denial of Q does require the denial of P.

But, again, it’s not really helpful to argue about this in this way.

The gricean interpretation of Ethan is that he intended to argue that he would deny the “exact opposite” of the assertion that Sagan also denies, but less strongly. That’s the distinction between Sagan’s contrary and Ethan’s contradictory.

I’m a little put-off by CB’s and Wow’s combativeness. There’s often ambiguity in language and in all cases not all native speakers will entirely agree on an intended meaning. That some speakers will interpret something differently than others — or, more to the point, that some speakers will find a statement confusing and contradictory until closer examination while others have not trouble with it at all — does not imply that one group of speakers are “right” and the other group is “wrong”.

It’s clear (after close consideration, for some) what Ethan intended and it’s equally clear that this is obvious to some but less obvious to others and for those the statement was initially confusing and inconsistent. I feel like this is an argument between people who claim that a joke is funny and people who claim that it isn’t, which is a pretty stupid argument.

It’s kinda sorta maybe interesting to be armchair analytical philosophers and try to rigorously parse what “exact opposite” precisely means in this context but, well, only if we don’t let that confuse us from understanding what is the simple intended meaning of what Ethan wrote, which doesn’t require such analysis. Ethan did not craft that statement as an analytic philosopher.

There’s a quite of Feynmans I modify when speaking about topics like this…

“There are all kinds of interesting questions that come from a knowledge of science, which only adds to the excitement and mystery and awe of a sunset”

That’s usually in response to someone trying to take me to task for deconstructing a beautiful event or item. But they see without knowing, and I believe a little understanding will make you appreciate certain things more and not less. Is the green flash any less amazing whe you understand the physics?

That said, Wow makes a logical fallacy, “denying the antecedent”. His argument can be understood as “If P, then Q therefore if not-P, then not-Q”, where “knowing a little” as P and “more romance” as Q:

Either wrong or you’re thinking of CB.

I’m arguing that

if C > A
then
giving someone A is a loss if you could have given them C

“I’m a little put-off by CB’s and Wow’s combativeness. There’s often ambiguity in language and in all cases not all native speakers will entirely agree on an intended meaning. ”

I’m put off by you saying this but then not actually taking it into account and treating Ethan’s statement as if there is a single interpretation to be logically parsed.

Language is ambiguous and open to multiple interpretations. Period. Legal documents attempt to remove this ambiguity, and can only partially succeed.

Everyone intuitively understands this. Everyone automatically interprets language by taking into account context. Holding multiple meanings in their head and selecting from them based on their applicability based on what context either already exists or is subsequently established. You’ve done it multiple times in this conversation with no need to even note it.

Ethan’s statement was ambiguous. Of the multiple possible interpretations of the sentence fragment, you picked one other than what he meant. The rest of the sentence establishes context which establishes which was intended. You didn’t realize that. Now you do. End of discussion.

Don’t become an Internet Pedant, who both denies and intentionally disables the part of the brain that handles this and calls that being “smart”.

I drive MA to OH several times a year. Last summer, I drove through the sunset into darkness but somehow passed it and got to see a second sunset. I was driving about 80 MPH. When I tell people this, they look at me like I am crazy! Got anything?

My first guess would be terrain. If you’re in a valley you would see the sun set over the top of the hill (if in OH) or mountain (if in the Appalachians), and the sky become dark as the parts still lighted by the sun aren’t visible to you. Then you crest the hill on the other side, and bam, there’s the sun.

Off over at The Graun, someone there has a quote from Feynman in 1981 that may be appropriate to repeat here:

“I have a friend who’s an artist and has sometimes taken a view which I don’t agree with very well. He’ll hold up a flower and say “look how beautiful it is,” and I’ll agree. Then he says “I as an artist can see how beautiful this is but you as a scientist take this all apart and it becomes a dull thing,” and I think that he’s kind of nutty. First of all, the beauty that he sees is available to other people and to me too, I believe. Although I may not be quite as refined aesthetically as he is … I can appreciate the beauty of a flower. At the same time, I see much more about the flower than he sees. I could imagine the cells in there, the complicated actions inside, which also have a beauty. I mean it’s not just beauty at this dimension, at one centimeter; there’s also beauty at smaller dimensions, the inner structure, also the processes. The fact that the colors in the flower evolved in order to attract insects to pollinate it is interesting; it means that insects can see the color. It adds a question: does this aesthetic sense also exist in the lower forms? Why is it aesthetic? All kinds of interesting questions which the science knowledge only adds to the excitement, the mystery and the awe of a flower. It only adds. I don’t understand how it subtracts.”

Of course, the answer to his query “I don’t understand how it subtracts.” is that religion has put us in primary position (below gods) and that those who buy into this think that if we AREN’T the sole candidates for the reason for all existence, then existence is pointless.

Rather more a problem with the “thought” processes of these religious people than with science or the reality it exposes.

The more I learn about the universe and the laws that govern it, the more beautiful and wonderous the nightsky is to me every night. This doesn’t belittle that I perceived maximum beauty before the start of my journey. Beauty is a relative not absolut measure and thus gaining its right value in the eyes of the regarder. No addition or substraction needed for it to grow, just a change of “relative location” in our lifetime of knowledge.

[…] The sky has many different colors and cool clouds that we like to see and take pictures of. The colors come from a reflection of the sun that makes the sky change to different colors. The more molecules, and particles in the sky the more the sky looks red and yellow. The clouds also act like a cool background with the color of the sky. Pictures of the sky has been happening for years now. (http://scienceblogs.com/startswithabang/2013/02/13/the-physics-of-sunsets/) […]